1. Field of the Invention
The present invention relates to the field of filament lamps and, more specifically, to incandescent or halogen lamps, especially of high voltage.
2. Discussion of the Related Art
FIG. 1 shows the conventional electric connection diagram of a lamp L, for example an incandescent lamp, to the electric network supplying an a.c. voltage Vac, for example of 230 volts RMS. Lamp L is connected in series with a turn on switch K between two terminals P, N that voltage Vac is applied to. Generally, switch K is interposed between phase terminal P of the single-phase a.c. voltage and lamp L. A protection device, generally a fuse or a circuit-breaker, of the electric panel of the installation is interposed between the line of the electric network and the circuit shown in FIG. 1.
Upon each lighting of the lamp by the closing of switch K, there appears a current surge, the instantaneous value of which is several times higher than that of the nominal operating current. The amplitude of the surge current peak varies according to the temperature of the filament of lamp L and to the instantaneous value of voltage Vac at the time the switch is closed. At low temperature and at the peak of a halfwave of voltage Vac, this surge current can reach up to fifteen times the current in the nominal state. The stress then undergone by the lamp filament reduce its lifetime. Filament lamps generally have a maximum lifetime on the order of 1000 hours.
FIG. 2 illustrates, in the form of timing diagrams, the shape of voltage VL across lamp L and of current IL in the filament of lamp L. In FIG. 2, a closing of switch K has been assumed to occur at a time t1. A peak of current IL appears at time t1 and this peak takes several halfwaves to damp so that the current in the lamp oscillates between nominal peak values Inom and -Inom.
This phenomenon is mainly linked to the variation of the value of the resistance of the filament of lamp L according to temperature. At low temperature, the filament resistance is lowest. The nominal power of the lamp is determined by the value of the filament resistance at high temperature, once the lamp has been lit. As a specific example, for a voltage Vac of 230 volts RMS., a 60 watt bulb has a filament resistance value on the order of 880 ohms at high temperature. At low temperature, if the value of the resistance is divided by 10, which corresponds to a usual proportion and if the lighting is performed at the peak of voltage Vac, that is, 340 instantaneous volts, the same lamp and the network line see an instantaneous power peak of more than 1200 watts.
In addition to the fact that these power peaks break most incandescent lamps, that is, they break the filament upon power-on, an arc may appear between a free end of the filament and the bulb cap, and this arc is likely to cause the fusing of the protection fuse involved, and thus installation servicing costs.
The same phenomenon can be observed in halogen lamps provided with a filament, in particular, for high-voltage halogen lamps, that is, lamps which are not supplied via a step-down transformer.
Another disadvantage of current peaks upon lighting of a filament lamp is that the disturbances on the lamp supply line are likely to damage other devices connected on the same circuit, that is, downstream of the same fuse.
A conventional solution to overcome these disadvantages is to provide, in series with the lamp, a negative coefficient thermistance, that is, a resistance, the value of which decreases with temperature. Such a solution has several disadvantages. First, the thermistance is prejudicial to the lighting installation since its resistance at high temperature remains non-negligible and thus results in power dissipation. Thermistances having very high low temperature values must indeed be chosen to sufficiently limit the current peak. As a result, the power effectively provided by the lamp does not correspond to its nominal value. Second, a thermistance cools down more slowly than the filament of an incandescent lamp. Accordingly, thermistance protection is inefficient in case of repeated lightings of the lamp, at short time intervals.
In other applications, it has already been provided to limit surge current peaks at the turning on of a device, by means of an electronic control circuit controlling the power-on so that it occurs at a zero crossing of the a.c. voltage. Such a solution is however not adapted to filament lamps. Indeed, although such a solution would reduce the current peak amplitude upon closing of switch K (time t1), the current in the lamp would however exceed the nominal values during several halfwaves and would then no longer be limited, during the time required by the filament to heat up sufficiently for its resistance to reach its nominal value. Further, the implementation of such a solution requires a complex electronic circuit requiring, most often, the generation of a low biasing voltage of the electronic components forming it.
The present invention aims at providing a novel solution to limit the surge current in a filament lamp.
The present invention aims, in particular, at providing a device which maintains the current in the lamp between the nominal values for which the lamp has been developed.
The present invention also aims at providing a device which is of simple constitution and of low cost.
The present invention also aims at providing power to the device without using specific power supply means.
The present invention further aims at providing a device of low bulk.
To achieve these and other objects, the present invention provides a circuit for limiting the surge current of a filament lamp, adapted to be connected in series between the filament and a switch that can supply an a.c. voltage, and including at least one controllable active element, for limiting the current to a predetermined threshold value.
According to an embodiment of the present invention, the threshold value is set by means of a resistor that is used to measure the current through the lamp.
According to an embodiment of the present invention, the circuit includes at least one limiting element in series with the measurement resistor, a control terminal of the limiting element being connected to a control means, detecting the voltage across the measurement resistor.
According to an embodiment of the present invention, the means is formed of a bipolar transistor, between the base and the emitter of which is connected the measurement resistor, the limiting element being controlled in linear mode.
According to an embodiment of the present invention, the means is formed of a comparator that compares the voltage across the measurement resistor with respect to a predetermined reference value, the limiting element being controlled in switched mode.
According to an embodiment of the present invention, the limiting element is biased, outside limiting periods, by a resistor connected between one of the power terminals of this element and its control terminal.
According to an embodiment of the present invention, the active element is connected as a unidirectional limiter and is associated with a rectifying bridge.
According to an embodiment of the present invention, the circuit includes two limiting elements, to limit the current in the lamp to the predetermined threshold value, each element being mounted as a unidirectional limiter.
According to an embodiment of the present invention, the circuit includes two field effect MOS transistors, connected in series between the switch and the filament, the measurement resistor being interposed between the two transistors and the current path including, upon each halfwave of the supply voltage, a parasitic diode of one of the two field effect transistors.
The present invention also relates to a filament bulb including, in its cap, a current limiting circuit.
The foregoing objects, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.